The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public LAW 96-517 (35 USC 202) in which the Contractor has elected to retain title.
The invention generally relates to organic fuel cells and in particular liquid feed organic fuel cells.
Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. In an organic/air fuel cell, an organic fuel such as methanol, formaldehyde, or formic acid is oxidized to carbon dioxide at an anode, while air or oxygen is reduced to water at a cathode. Fuel cells employing organic fuels are extremely attractive for both stationary and portable applications, in part, because of the high specific energy of the organic fuels, e.g., the specific energy of methanol is 6232 Wh/kg.
Two types of organic/air fuel cells are generally known:
1. An xe2x80x9cindirectxe2x80x9d or xe2x80x9creformerxe2x80x9d fuel cell in which the organic fuel is catalytically reformed and processed into carbon monoxide-free hydrogen, with the hydrogen so obtained oxidized at the anode of the fuel cell.
2. A xe2x80x9cdirect oxidationxe2x80x9d fuel cell in which the organic fuel is directly fed into the fuel cell without any previous chemical modification where the fuel is oxidized at the anode.
Direct oxidation fuel cells do not require a fuel processing stage. Hence, direct oxidation fuel cells offer a considerable weight and volume advantage over the indirect fuel cells. Direct oxidation fuel cells use either a vapor or a liquid feed of the organic fuel. Current art direct oxidation fuel cells that have shown promise typically employ a liquid feed design in which a liquid mixture of the organic fuel and a sulfuric acid electrolyte is circulated past the anode of the fuel cell.
The use of sulfuric acid electrolyte in the current-art direct methanol fuel cells presents several problems. The use of sulfuric acid, which is highly corrosive, places significant constraints on the materials of construction of the fuel cell. Typically, expensive corrosion resistant materials are required. Sulfate anions, created within the fuel cell, have a strong tendency to adsorb on the electrocatalyst, thereby hindering the kinetics of electro-oxidation of the fuel and resulting in poor performance of the fuel electrode. Also, sulfuric acid tends to degrade at temperatures greater than 80xc2x0 C. and the products of degradation usually contain sulfur which can poison the electrocatalyst. In multi-cell stacks, the use of sulfuric acid electrolyte can result in parasitic shunt currents.
Exemplary fuel cells of both the direct and indirect types are described in U.S. Pat. Nos.: 3,013,908; 3,113,049; 4,262,063; 4,407,905; 4,390,603; 4,612,261; 4,478,917; 4,537,840; 4,562,123; and 4,629,664.
U.S. Pat. Nos. 3,013,908 and 3,113,049, for example, describe liquid feed direct methanol fuel cells using a sulfuric acid electrolyte. U.S. Pat. Nos. 4,262,063, 4,390,603, 4,478,917 and 4,629,664 describe improvements to sulfuric acid-based methanol fuel cells wherein a high molecular weight electrolyte or a solid proton conducting membrane is interposed between the cathode and the anode as an ionically conducting layer to reduce crossover of the organic fuel from the anode to the cathode. Although the use of the tonically conducting layer helps reduce crossover, the ionically conducting layer is used only in conjunction with a sulfuric acid electrolyte. Hence, the fuel cell suffers from the various aforementioned disadvantages of using sulfuric acid as an electrolyte.
In view of the aforementioned problems associated with using sulfuric acid as an electrolyte, it would be desirable to provide a liquid feed fuel cell that does not require sulfuric acid as an electrolyte.
In addition to the improvements in operational characteristics of the liquid feed fuel cell, the conventional method of fabricating high-surface-area electro-catalytic electrodes for use such fuel cells also needs to be improved. The existing method of fabrication of fuel cell electrodes is a fairly time-consuming and expensive procedure. Specifically, electrode fabrication requires that a high surface-area carbon-supported alloy powder be initially prepared by a chemical method which usually requires about 24 hours. Once prepared, the carbon supported alloy powder is combined with a Teflon(trademark) binder and applied to a carbon fiber-based support to yield a gas diffusion electrode. To volatilize impurities arising out of the Teflon(trademark) binder and to obtain a fibrous matrix of Teflon(trademark), the electrodes are heated to 200-300xc2x0 C. During this heating step, oxidation and sintering of the electrocatalyst can occur, resulting in a reduced activity of the surface of the electrode. Thus, the electrodes often require re-activation before use.
Also electrodes produced by conventional methods are usually of the gas-diffusion type and cannot be effectively used in liquid feed type fuel cells as the electrode is not adequately wetted by the liquid fuel. In general, the structure and properties of a fuel oxidation electrode (anode)for use in liquid feed type fuel cells are quite different from the gas/vapor feed fuel cells such as the hydrogen/oxygen fuel cell. The electrode structures for use in a liquid feed fuel cell should be very porous and the liquid fuel solution should wet all pores. Carbon dioxide that is evolved at the fuel electrode should be effectively released from the zone of reaction. Adequate wetting of the electrodes is a major problem for liquid feed fuel cellsxe2x80x94even for those which use a sulfuric acid electrolyte.
As can be appreciated, it would be desirable to provide improved methods for fabricating electrodes, particularly for use in liquid feed fuel cells. It is also desirable to devise methods for modifying electrodes, originally adapted for gas-feed fuel cells, for use in liquid feed fuel cells.
In addition to improving the liquid feed fuel cell itself and for providing improved methods for fabricating the electrodes of fuel cell, it would be desirable to provide new effective fuels as well. In general, it is desirable to provide liquid fuels which undergo clean and efficient electro-chemical oxidation within the fuel cell. The efficient utilization of organic fuels in direct oxidation fuel cells is, in general, governed by the ease by which the organic compounds are anodically oxidized within the fuel cell. Conventional organic fuels, such as methanol, present considerable difficulties with respect to electro-oxidation. In particular, the electro-oxidation of organic compounds such as methanol involves multiple electron transfer and is a very hindered process with several intermediate steps. These steps involve dissociative adsorption of the fuel molecule to form active surface species which undergo relatively facile oxidation. The ease of dissociative adsorption and surface reaction usually determines the facility of electro-oxidation. Other conventional fuels, such as formaldehyde, are more easily oxidized, but have other disadvantages as well. For example, formaldehyde is highly toxic. Also, formaldehyde is extremely soluble in water and therefore crosses over to the cathode of the fuel cell, thus reducing the performance of the fuel cell. Other conventional organic fuels, such as formic acid, are corrosive. Furthermore, many of the conventional organic fuels poison the electrodes of the fuel cell during electro-oxidation, thus preventing sustained operation. As can be appreciated, it would be desirable to provide improved fuels, particularly for use in liquid feed fuel cells, which overcome the disadvantages of conventional organic fuels, such as methanol, formaldehyde, and formic acid.
A general object of the invention is to provide an improved direct type liquid feed fuel cell. One particular object of the invention is to provide a direct type liquid feed fuel cell which does not require a sulfuric acid electrolyte. Another particular object of the invention is to achieve adequate wetting of electrodes for use in liquid feed fuel cells. Yet another particular object of the invention is to provide an improved method for wetting electrodes for use in fuel cells having sulfuric acid electrolytes. Still another particular object of the invention is to provide improved fuels for use in liquid feed fuel cells.
The object of providing an improved liquid feed direct fuel cell which does not require a sulfuric acid electrolyte is achieved in part by using a solid polymer electrolyte membrane in combination with a battery-type anode that is porous and is capable of wetting the fuel. In the improved liquid feed fuel cell, a battery-type anode structure and a cathode are bonded to either side of the solid polymer proton-conducting membrane forming a membrane-electrode assembly. A solution of methanol and water which is substantially free of sulfuric acid is circulated past the anode side of the assembly.
A solid polymer membrane is used, in part, because such membranes have excellent electrochemical and mechanical stability, high ionic conductivity, and can function both as an electrolyte and as a separator. Also, the kinetics of electro-oxidation of methanol and electro-reduction of air or oxygen are more facile at an electrode/membrane-electrolyte interface as compared to an electrode/sulfuric acid interface. The use of the membrane permits operation of the fuel cell at temperatures as high as 120xc2x0 C. Since the fuel and water solution is substantially free of sulfuric acid, there is no need for expensive corrosion-resistant components in the fuel cell and its accessories. Also the absence of conducting ions in the fuel and water solutions, which can exist when a sulfuric acid electrolyte is employed, substantially eliminates the possibility of any parasitic shunt currents in a multi-cell stack.
The solid polymer electrolyte is preferably a proton-conducting cation-exchange membrane, such as the perflourinated sulfonic acid polymer membrane, Nafion(trademark). Nafion(trademark) is a copolymer of tetrafluoroethylene and perfluorovinylether sulfonic acid. Membranes of modified perflourinated sulfonic acid polymer, polyhydrocarbon sulfonic acid and composites of two or more kinds of proton exchange membranes can also be used.
The anode is preferably formed from high surface area particles of platinum-based alloys of noble and non-noble metals. Binary and ternary compositions can be used for the electro-oxidation of organic fuels. Platinum-ruthenium alloy, with compositions varying from 10-90 atom percent of platinum, is the preferred anode electrocatalyst for the electro-oxidation of methanol. The alloy particles are either in the form of fine metal powders, i.e., xe2x80x9cunsupportedxe2x80x9d, or are supported on high surface area carbon material.
Conventional fuel cell anode structures (gas diffusion type) are not suitable for use in liquid feed type organic/air fuel cells. These conventional electrodes have poor fuel wetting properties. These conventional electrodes can be modified for use in liquid feed type fuel cells by coating them with substances that improve their wetting properties. Nafion(trademark) with an equivalent weight of 1000 or higher is the preferred substance. The additive decreases interfacial tension of the liquid/catalyst interface and leads to the uniform wetting of the electrode pores and particles by the fuel and water solution, yielding enhanced utilization of the electrocatalyst. In addition to improving wetting properties, Nafion(trademark) additive also provides ionic continuity with the solid electrolyte membrane and permits efficient transport of protons or hydronium ions generated by the fuel oxidation reaction. Further, the additive facilitates the release of carbon dioxide from the pores of the electrode. By using a perfluorinated sulfonic acid as the additive, anionic groups are not strongly adsorbed on the electrode/electrolyte interface. Consequently, the kinetics of electro-oxidation of methanol are more facile than in sulfuric acid electrolyte. Other hydrophilic proton-conducting additives with the desired properties include montmorrolinite clay, alkoxycelluloses, cyclodextrins, mixtures of zeolites, and zirconium hydrogen phosphate.
The object of improving electrodes for operating in liquid feed fuel cells is achieved, in part, by using perfluorooctanesulfonic acid as an additive in an electro-deposition bath used in fabricating the electrode. An electro-deposition method using the perfluorooctanesulfonic acid additive comprises the steps of positioning a high-surface-area carbon electrode structure within a bath containing metallic salts, positioning an anode within the bath and applying a voltage between the anode and the cathode until a desired amount of metal becomes deposited onto the electrode. After deposition of the metal onto the electrode, the electrode is extracted from the bath and washed within deionized water.
Preferably, the metal salts include hydrogen hexachloroplatinate and potassium pentachloroaquoruthenium. The anode is composed of platinum. The carbon electrode structure includes high-surface-area carbon particles bound together by polytetrafluoroethylene, sold under the trademark Teflon(trademark).
The object of providing for adequate wetting of an electrode within a liquid feed fuel cell having a sulfuric acid electrolyte is achieved by employing perfluorooctanesulfonic acid as an additive to the fuel mixture of the fuel cell. Preferably, the perfluorooctanesulfonic acid is added to the organic fuel and water mixture in concentrations from 0.001-0.1 M.
The general objective of providing new fuels for use in organic fuel cells is achieved by using either trimethoxymethane, dimethoxymethane or trioxane. All three new fuels can be oxidized at a high rate into carbon dioxide and water within the fuel cell without poisoning the electrodes. Furthermore, neither trimethoxymethane, dimethoxymethane or trioxane are corrosive. Rates of oxidation of the three new fuels are comparable to, or better than, oxidation rates of conventional organic fuels. For example, rates of oxidation for dimethoxymethane are higher than that of methanol, at the same temperature. Trioxane achieves oxidation rates comparable to that of formaldehyde. However, trioxane has a much higher molecular weight than formaldehyde and, as such, molecules of trioxane do not cross-over to the cathode of the fuel cell as easily as molecules of formaldehyde.
Trimethoxymethane, dimethoxymethane and trioxane may be employed in a fuel cell having any of the improvements set forth above. However, the improved fuels may also be advantageously used within other organic fuel cells, including entirely conventional fuel cells.
Hence the various general objects of the invention set forth above are achieved. Other objects and advantages of the invention will be apparent from the detailed description set forth below.